Electromechanical wave filter



4, 1939- H. e. OCH 2,164,541

ELECTROMECHANICAL WAVE FILTER Filed May 14, 1938 INVENTOR 8y H G. OCH

A 7'7'ORNEY Patented July 4, 1939 UNITED STATES PATENT OFFICE ELECTROMECHANICAL WAVE FILTER Application May 14, 1938, Serial No. 207,928

4 Claims.

This invention relates to improvements in electro-mechanical wave filters.

An object of the invention is to improve the efficiency of such filters.

A further object is to provide more economical means for eliminating the effect of unwanted mutual inductance between the coils of such a filter and providing an appropriate termination at one end for parallel operation with filters transmitm ting adjacent frequency bands.

Another object is to reduce the leakage flux between the coils of such a filter and thus eliminate the resulting dissipation of energy.

Other and further objects will become apparent 15 during the course of the following description.

This invention is directed to electromechanical filters of the general types illustrated in the copending application of E. Lakatos, Serial No. 101,061, filed September 16, 1936, and particularly of the type employing essentially the structure illustrated in Fig. 3 of that application. In this latter type of electromechanical filter it has been found that objectionable dissipation results from the leakage of flux between the two coils of the 25 device. In accordance with the present invention the coils are divided into equal parts and corresponding parts of the two coils are assembled adjacent to each other so that leakage of fiux is substantially eliminated.

30 An additional feature of this invention is its applicability to the situation where the transmission characteristics desired require that the mutual inductance of the windings be annulled and in addition that one end of the filter be X termi- 35 nated for parallel operation with one or more similar filters passing adjacent frequency bands. The advantages resulting from employing X terminations for wave filters for parallel operation are explained in U. S. Patent 1,557,230, issued 40 October 13, 1925, to O. J. Zobel. As is well known to the art X-termination consists in changing the terminating arm of the network so that its impedance is a particular fractional part of the normal impedance of a corresponding full arm 45 of the network being terminated. For example, if a network has a full series arm impedance of Z1, it would be X terminated by a series arm having an inductance of XZi, X being a fraction less than unity. The value of X is chosen in accordance with principles described at length in the above-mentioned patent to Zobel.

While each of the above-described requirements could be satisfied by employing two independent retardation coils, one to annul the ung wanted mutual and the other to provide the desired fractional termination of the filter, it is possible in accordance with the present invention to employ a single coil having two windings on a common core to accomplish both results. As the cost of such a coil or transformer is only slightly 5 more than that of a single retardation coil, an appreciable economy is thereby eifected.

The features of the invention will be more fully understood from the following detailed description and by reference to the accompanying draw ing of which:

Fig. 1 shows the assembly of an electromechanical filter of this invention with portions of the coils, core and supporting frame broken away to show the construction more clearly;

Fig. 2 shows in diagrammatic form an equivalent network of a filter of the type illustrated in Fig. 1;

Fig. 3 shows in schematic form an equivalent electrical network of the device of Fig. 1; 0

Fig. 4 is illustrative of the magnetic flux paths established if the coils are not subdivided and corresponding sections mounted adjacently in the manner indicated in Fig. 1;

Fig. 5 shows in schematic form a combination of the device of Fig. l with additional series condensers and a shunt coil to widen and improve the transmission band of the electromechanical filter of Fig. l; and

Fig. 6 is illustrative of the further feature of this invention whereby with a single auxiliary two-winding coil the combination of Fig. 5 may be given an X-termination and at the same time the mutual inductance between the filter coils themselves may be annulled.

In detail, with particular reference to Fig. 1, the electromechanical portions of filters of this invention may comprise a frame of non-magnetic material l0, arranged to support a core of magnetic material I I of low reluctance to alternating fiuX, having a single small gap in which the end of an armature piece l7 may vibrate. Armature ll is supported on spring member 23 which in turn is clamped between the halves of block l8.

An L-shaped permanent magnet l2, one end being keyed to fit block 18, and the other drilled and threaded to be joined firmly to the center of the rear side of core II, is supported at its elbow by a projection 2| of frame l6 and block I8 is clamped against magnet 12 by screw [9.

By virtue of this arrangement the steady flux of permanent magnet l2 passes through the left half of core H in the opposite direction from that in which it passes through the right half of core II. The distribution of direct flux to the two halves of core 1 I obviously depends upon the position of the armature and varies in accordance with changes in position resulting from vibration of the mechanical vibrating system. The permanent magnet 12 having high reluctance to alternating flux, does not sensibly reduce the alternating flux flowing through core ll. Core II is laminated to eliminate eddy current losses and the tip 22 of armature I? may, for the same reason, be of suitable compressed powdered magnetic material, the particles being insulated by lacquer or other suitable binding material.

As explained in United States Patent 2,115,818 to E. Lakatos, issued May 3, 1938, an electromechanical device such as that of Fig. 1 in which an electrical input circuit is coupled through a simple mechanical Vibratory system to an electrical output circuit, may be represented diagrammatically as indicated in Fig. 2 of this application. (Refer particularly to Fig. 13 of United States Patent 2,115,818 to Lakatos where, omitting the electrical condensers S1 and S2, the structure contemplated is shown.)

In Fig. 2, L1 represents the input winding comprising half windings l3 and M of Fig. 1, L2 represents the output winding comprising half windings l5 and [6 of Fig. 1, M represents the electromagnetic coupling between these windings, G1 represents the force factor which provides the respective electromagnetic couplings between the mechanical vibratory system and the above-mentioned windings L1 and L2, and mass M and stiifness K represent the mass and stiffness respectively of the bar armature I! of Fig. 1 and the spring 23 of Fig. 1, comprising the mechanical vibratory system.

Assuming that the device of Fig. 1 is substantially symmetrical about a vertical plane which longitudinally bisects the permanent magnet 12 and that the input and output windings are equal (that is, that in Fig. 2, inductance L1 may be considered equal to inductance L2) and the force factor coupling L1 to the vibratory system may be considered equal to the force factor coupling L2 thereto, we may write the following mesh equations for the structure of Fig. 2.

o=i pL +i pZW +G v (3) where p=j21rf, 2'1 and 12 are the respective mesh currents of the input and output circuits and o is the velocity of the vibrating member; from (2) Equations (5) and (6) may obviously be represented by a two-mesh electrical network, as shown in Fig. 3, from which it is evident that the structure of Fig. 1 may be represented by a simple T-network of inductances as shown in Fig. 3 in terms of the self-inductance L1 and mutual inductance Mn of Fig. 2 and an antiresonant combination in each arm of the network as shown in Fig. 3 in terms of the force factor G1, the mass M and the stiffness K of Fig. 2.

In the series arms the anti-resonant combinations represent the respective reactions upon the coils L1 and L2 of the vibrating system and the anti-resonant combination in the shunt arm represents the coupling between coils L1 and L2 which is afforded by the vibrating system.

The elements of the equivalent network representing the coupling between the input and output coils are conventionally represented as negative and an electrical network as illustrated in Fig. 3 would have the same transmission, phase and impedance characteristics as the device of Fig. 1 employing the winding comprising sections [3 and Id connected in series aiding as the input and the winding comprising sections I5 and it connected in series aiding as the output. Physically the two windings of Fig. 1 (each comprising two sections as described in detail above) are connected in series aiding also and the junction between the two windings is employed as the third or ground terminal of the T-network.

The reason for dividing the input and output windings of the device of Fig. 1 into halves and assembling half of each winding on each side of the air-gap is apparent from the diagram of Fig. 4 which represents the flux paths which would obtain were the coils not subdivided. In Fig. 4, coils A6 and 48 are assembled on core M, the coils being on opposite sides of an air-gap in which an armature 1! is disposed. It is obvious that appreciable flux from each of coils Q6 and 48 would not thread the other coil because of the intervening air-gap but would be merely what is commonly termed stray flux and is accompanied by loss of energy and efiiciency.

The structure of Fig. 3 is by inspection in the light of well-known filter design theory, a bandpass wave filter. Upon closer inspection, however, it is found that only a relatively narrow band of frequencies may be passed through a filter having the configuration of Fig. 3 unless the inductance Mo of the shunt arm is effectively removed. Since it is negative it may be removed by placing effectively in series with it an equal positive inductance. The transmission characteristics of the filter may be still further improved by adding in each series arm a series condenser.

These additions may be efiected by the use of auxiliary electrical elements and lead to a combination having an equivalent electrical network as shown in Fig. 5.

Physically the combination indicated in Fig. 5 consists of a device, as illustrated in Fig. 1, two auxiliary condensers and one auxiliary inductance coil, the latter three elements being of any convenient conventional design. These auxiliary elements may conveniently be mounted on a common base with the device of Fig. 1. The two windings of the device of Fig. 1 are, as described above, connected in series aiding. The auxiliary inductance coil, being designed to annul the mutual inductance between these two windings, must be connected at one terminal to the junction between them. The free terminal of the auxiliary coil then becomes the third or ground terminal of the T-network. Inductance M represents this auxiliary coil and terminals 21 represent the ground terminal in Fig. 5. The free terminals of the windings of the device of Fig. 1 are then connected to the two auxiliary condensers :1 .the shunt arm as described above.

represented by capacities 30 and 38, respectively, of Fig. 5. The free terminals of the auxiliary condensers then become the input and output terminals, respectively, of the T-network represented by terminals 28 and 29 of Fig. 5. This combination may in accordance with well-known filter theory provide a wide band wave filter.

If a wave filter having an equivalent electrical network, as shown in Fig. 5, is to be connected at one end in parallel with a similar filter or filters transmitting adjacent frequency bands, the final arm of the network should, as described in the above-mentioned patent to O. J. Zobel, be given a particular fractional portion of the normal full arm impedance. This is commonly known as X- termination of the filter.

For the network of Fig. 5, assuming it is desired to operate it in parallel with one or more similar filters at the terminals 28 and 21, it will be necessary to modify condenser 30 and to add an auxiliary inductance coil in series therewith. It should. be noted that inductance 33 of Fig. 5 is contributed by the input winding of the device of Fig. 1, and this winding may not be altered without effectively changing the impedance of the output circuit, because the input and output windings are electromagnetically coupled. However, one auxiliary inductance coil is already required to provide the inductance 4i employed in By design in accordance with the following formulae, the inductance required to provide the desired X-termination may be obtained from the same auxiliary inductance coil as is employed to furnish inductance ii. The two are, in efiect, built into a transformer, the two windings of which finnish the desired auxiliary inductances for their respective arms of the filter network and the mutual between the two windings of this auxiliary coil is taken care of by reproportioning the other elements of the filter. By this means the use and substantially the entire cost of a second independent auxiliary inductance coil may be avoided, since an auxiliary two-winding coil of the type contemplated may be constructed at only slightly greater cost than a single-winding coil. This leads to the combination represented by the equivalent electrical network of Fig. 6. The auxiliary two-winding coil above mentioned is represented in the schematic diagram of Fig. 6 by inductances 52 and 55 coupled by mutual inductance Mat.

The formulae employed in the conversion are as follows:

In the above equations, L1 is the inductance and C1 is the capacity required in a mid-Series arm comprising a series resonant combination of a coil and a condenser, for a confluent band filter having the desired band width, frequency location and impedance. These elements, as is well known in the art, are defined in terms of the characteristic impedance Z0, the cut-off frequencies f1 and f2 and the mid-band frequency ,fm.

Similarly L2 is the inductance and C2 is the capacity required in a mid-shunt arm (2Z2) comprising a parallel anti-resonant combination of a coil and a condenser, fora confluent band filter having the same desired band width, frequency location and impedance. These elements are also defined in terms of Z0, f1, f2 and m.

Ca of Equation (11) represents the modification, required for the series condenser in the input arm of the filter to contribute toward the attainment of the desired X-termination, as described in the above-mentioned patent to Zobel. In the equivalent electrical circuit of Fig. 6, Ca is condenser 5|.

Cb is condenser 6| in the output series arm of Fig. 6.

M of Equation (13) is the effective mass of the vibrating reed at the center of the air-gap. G1 is the force factor expressing the coupling between the reed and the electrical coils of the system.

K of Equation (14) is the of the reed.

Lo of Equations (15 to 17) inclusive, is the total inductance of each of the full windings (input and output) of the assembly of Fig. 1.

M0 is the coupling between the input and output windings of Fig. 1. La and Lb are the selfinductances of the two compensating coil windings of the above-mentioned auxiliary two-winding coil represented by inductances 52 and 66, respectively, in the diagram of Fig. 6 and Mab is the coupling between these two windings.

By way of example, for an experimental combination of the nature contemplated in Fig. 6 designed in accordance with the principles of this invention, the values of the above items were approximately as follows:

OhnlS f =1275 cycles per second f f =llO cycles per second L1=.868 henries c =.0l microfarads Lg=.00657 henries C =2A1 microfarads C =.0112 microfarads C =.0180 microfarads G =3.34 x 10 dynes M=5.38 grams K=.00290 centimeters per dyne M per cent M =98 per cent L =.561 henries L =.132 henries L =.307 henries.

The physical dimensions of the magnetic core efiective compliance were, approximately, length 2 inches, breadth 1 4 inches, cross-sectional area .063 square inch, length of air-gap .288 inch. It was made of an alloy comprising approximately per cent nickel, 4 per cent molybdenum and the balance iron. The reed was of cold rolled steel by byl inches, except for its tip, in the air-gap, which was of insulated powder of the alloy employed for the core. The tips dimensions were by A; by inch. The spring was of cold rolled steel and its dimensions were inch wide by inch high by .105 inch thick.

The permanent magnet was of a steel alloy comprising approximately 36 per cent cobalt, '7 per cent tungsten, 4 per cent chromium, .6 to .8 per cent carbon and the balance iron, and was magnetized to saturation in place. Its crosssection was .263 square inch.

Each half of the input and output windings, respectively, comprised 1300 turns of number 29 wire. The auxiliary two-winding coil, represented in the schematic diagram of Fig. 6 by inductances 52 and 66 coupled by mutual inductance Mas as mentioned above, comprised windings of 1250 turns of number 34 wire and 1900 turns of number 34 wire, respectively, on a toroidal core of insulated powder of the alloy employed for the reed tip as above described. The coil core was of .075 square inch cross-section and 1.13 inches mean diameter.

Numerous applications of the principles of this invention will occur to those skilled in the art. No attempt has here been made to be exhaustive. The scope of the invention is defined in the following claims.

What is claimed is:

1. An electromechanical transducer comprising a mechanical vibratory member, a magnetic core of low reluctance to alternating flux providing an air-gap in which one end of said vibratory member is positioned, a source of direct magnetomotive force associated with said vibratory member, a magnetic path of low reluctance to direct flux including said source of direct magnetomotive force and connecting said vibratory member and the magnetic mid-point of said core, and an input winding and an output winding assembled on said core, said windings being connected in series aiding and the point of junction between the said input and output windings being connected to the grounded side of the transducer circuit, each of said windings being subdivided into a plurality of parts connected in series, the parts of one winding being electrically balanced with correspond ing parts of the other Winding, said corresponding parts of the two windings being mounted on said core adjacent to each other whereby leakage flux between the two windings is substantially eliminated and the efficiency of said transducer is increased.

2. In combination, an electromechanical transducer as defined in claim 1 and an electrical transformer having two windings, one of said windings being connected in series with the transducer input and the other between the point of junction of the input and output windings and the grounded side of the transducer circuit, the self-inductances of the windings of said transformer and the mutual inductance between them being so adjusted that the mutual inductance between the transducer input and output windings is effectively annulled and at the same time the input impedance of the transducer is modified for more efiicient parallel operation with electrical- 1y similar transducers passing frequency bands adjacent to that passed by the combination of this claim.

3. An electromechanical transducer as in claim 1, the end of the vibratory member positioned in the air-gap of the core being made of particles of magnetic material bound together by an insulating binding material whereby eddy currents will not be established therein by the alternating flux and increased efliciency of the transducer is obtained.

4. An electromechanical wave filter comprising a magnetic core of low reluctance to alternating current flux, said core having an air-gap therein, an input winding and an output winding, said input and said output windings being disposed on said core, a half of each of said windings being disposed on each side of the said airgap in said core, means for introducing direct flux into said core, said means comprising a source of direct flux and a frequency-selective mechanical vibratory system, a portion of said vibratory system being disposed in said air-gap, said vibratory system being responsive to alterhating flux within a desired range of frequencies whereby when alternating currents within said desired range of frequencies are introduced into said input winding said vibrating system will respond and cause variations in the said direct flux in said core corresponding to said alternating input currents.

HENRY G. OCH. 

